The detection of special nuclear material (SNM) at a large stand-off distance has application in such fields as nuclear security and counter-terrorism. At a given standoff distance a detector can utilize detection of either neutrons or gamma-rays as a signature of the nuclear material due to the respectively high penetrating power of the neutrons or gamma-rays. Compared with gamma-rays, neutrons (e.g., fast neutrons) have a background that is relatively low and understood, which can provide an ideal regime for detection of nuclear material in large search area scenarios.
Currently there are two main approaches for generation of a neutron imager. In a first approach, a coded aperture can be based on a front pixelated mask plane containing hydrogenous material in combination with a detector array as the rear plane. The pattern of the mask plane can be based on a uniformly redundant array (URA) which has a mathematical property to facilitate unique reconstruction of the spatial distribution of a source under ideal imaging conditions. Additionally, an optimized random mask can also be utilized. However, in practice, the construction and calibration of a large detector array can impact the cost and performance of such detectors.
In a second approach, the front mask plane can be replaced with another large detector plane and accordingly identifies coincidence scattering in the front and rear plane. A device utilizing such technology is known as a scatter camera. In this approach, the direction of an incoming neutron can be constrained and hence maps out a source distribution. Such an approach has been shown to be successful in near and medium distance regimes, however the requirement of double scatters can greatly reduce system efficiency and further can increase a dwell time necessary for long-range detection.
Time-encoded imaging (TEI) is a recently developed method for SNM detection with neutrons that overcomes the limiting factors of the two main approaches. The concept relies on measuring the presence of a nuclear source as a time varying neutron rate as it is modulated by shielding material moving around each detector. TEI designs feature simple construction reducing cost and complexity while maintaining a high efficiency by requiring only single scatters. Additionally by converting spatial variations into temporal rates the system is more robust against non-uniformities in the detected background due to detector geometry. For these reasons TEI detectors offer simple, low-cost, and highly efficient solutions for detecting SNM. Another approach utilizes Rotating Modulated Collimators (RMC). However, while incorporating some of the advantages of a TEI design, the RMC approach is unsuitable for neutron detection owing to the required thick masks significantly reduce the efficiency of the imager.